Good study habits start from a young age. Study doesn't necessarily have to be maths sums or spelling when they are young instead it can be a quiet time without television or computer games. For example when a five year old comes home from school get them into the habit of sitting down with a book or listen to some music for fifteen minutes. Call it quiet time rather than study. Once a child has begun to read you can use this fifteen minutes quiet time to practise reading.
As a child gets older you can lengthen the time which will happen naturally as they get spellings and homework. Reward the child with time on the computer or with their favourite television programme. Make television or the computer a treat rather than a habit as they come in from school.
Children focus better when they have had a good play outside. Before your quiet time or study make sure they let off some steam and then calm them down with a drink (not fizzy or sugary) and maybe a small snack. Remember you need a calm atmosphere and some foods can trigger restlessness so avoid those sugar and additive packed foods before they need to study or be quiet.
Routines work well for children so make study or quiet time a set time of the day. Children need to know where they stand and if they know each day at a certain time they need to study there will be less confrontation. Be firm and stand your ground but don't turn it into an issue otherwise you will never get them to settle.